Coral Reefs: Diversity on Display

Grey Reef Shark

Best known for its spectacular agonistic display, the Grey Reef Shark (Carcharhinus
amblyrhynchos) is pugnacious all out of proportion to its modest size.
The agonistic display in this species ranges from mild to extreme, depending
upon degree of escape route restriction. This display is elicited in
approach-withdrawal conflict situations and appears to be ritualized from
movements and postures associated with overpowering prey. It features
rolling or spiraling during laterally exaggerated swimming while lowering
the pectoral fins, raising the snout, and arching the back. The display is
highly stereotyped and seems to communicate that the shark ‘feels’
threatened — warning the receiver to “come no closer”. If the receiver of
this signal does not move away, the displaying shark may launch a
lightning-fast, slashing attack.

Despite such seemingly anti-social behavior, Grey Reef Sharks are
apparently highly social with members of their own species. Day-time
grouping behavior in Grey Reef Sharks has been investigated at Enewetak
Atoll, Marshall Islands, and found to consist of three basic patterns.
Polarized schools of 27 or more individuals occur close to the bottom over
level areas, with clusters of closely-spaced, parallel-oriented sharks
sometimes coalescing to form loose aggregations that together perform a
circular milling pattern. These schools seem to become larger as the day
wears on, suggesting that they break apart at night and gradually reform the
following day. Loose aggregations of 5 to 20 individuals occur near reef
drop-offs — places where, on the seaward side of the Atoll, the bottom falls
away abruptly from 15 to 200 feet (95 to 60 metres). Lastly, lone
individuals were usually observed over shallower reefs and lagoon pinnacles;
these solitary animals are not merely patrolling the reef aimlessly, for
they often make rapid investigatory approaches toward divers that sometimes
culminate in an agonistic display.

Grouping behavior in Grey Reef Sharks has also been studied at Johnston
Atoll, in the Central Pacific. Here, between March and late May, researchers
discovered that loose aggregations of about 30 sharks form during the
hottest part of each day. All the sharks are mature females, suggesting that
these aggregations may have something to do with reproduction. If most or
all of these sharks are pregnant, aggregating in warm water may increase the
rate of embryonic development. Alternatively, warm water at the aggregation
area may increase the growth rate of female sharks, thereby accelerating the
onset of maturity and possibly increasing the reproductive fitness of
individuals. Perhaps these female-only aggregations may simply be a
designated place to avoid amorous, nippy males during the reproductive
season. Or, since many species of sharks segregate by sex during much of the
year, these aggregation areas may function as a central landmark to foraging
routes, much like Scalloped Hammerheads (Sphyrna lewini) use
seamounts as refuging areas. Time and further research may shed additional
light on why these aggregations of female Grey Reef Sharks occur annually at
Johnston Atoll.

Activity rhythms and home range of Grey Reef Sharks have been studied via
sonic telemetry and direct observation at Rangiroa, French Polynesia. During
the day, these sharks typically swim slowly in groups, milling about a
well-defined “core area” in a relatively deep part of their collective home
range. At dusk, they move away from the relatively small core area and range
widely over shallower parts of the reef, presumably foraging. Telemetry data
also suggests that Grey Reef Sharks are more dispersed at night, but this
could not be confirmed by direct observation. For one Tiputa Pass individual
tracked continuously for 72 hours, its home range was about 0.3 square miles
(0.8 square kilometres) — an area slightly smaller than the average
nocturnal activity space of Whitetip Reef Sharks (Triaenodon obesus). This
smaller activity space of Grey Reef Sharks may reflect the richer feeding to
be had at the depths and parts of the reef favored by this species.
Dominated by Grey Reef Sharks, Whitetip Reef Sharks may be relegated to
foraging over suboptimal parts of the reef at night, imposing a need to
travel farther afield to secure sufficient food. As in human society, high
social rank among sharks has its privileges.

Grey Reef Sharks from Rangiroa Atoll show circadian rhythms that are
quite regular from day-to-day and, once learned, are quite predictable.
Their daily routines are apparently keyed to periodic environmental cues.
Movement patterns of Grey Reefs seem to be markedly influenced by the tidal
cycle, at least in areas of strong tidal currents and water clarity
fluctuations. Although there are slight regional differences in daily
activity patterns of these sharks from different areas of Rangiroa, their
circadian rhythms are fundamentally similar because they all respond in the
same way to the twice-daily pulse of the tides: moving shoreward during each
ebb tide and seaward during each flood.

Moving inshore and offshore counter to the tides may be related to Grey
Reef Shark feeding. By swimming against tidal currents, these sharks may be
better able to detect the scent of dead or injured animals inside lagoons or
to take predatory advantage of additional cover provided by turbid, moving
water. Although less speedy than the open-ocean Shortfin Mako (Isurus
oxyrinchus), the Grey Reef Shark is much more maneuverable in ‘hot
pursuit. In the complex architecture of the coral reef, inhabited by
uncountable millions of small and agile fishes, maneuverability may be more
advantageous than flat-out sprinting speed. From its highly varied diet,
there can be little doubt that the Grey Reef Shark is a formidable apex
predator on Indo-Pacific coral reefs.

Regardless of their relative sizes, Grey Reef Sharks are dominant over
most other sharks with which they share Indo-Pacific coral reefs. The highly
stereotyped nature of the Grey Reef Shark’s agonistic display suggests this
behavior is probably important in normal social encounters with sharks of
the same and other species. There is, for example, a report from Rangiroa of
a Grey Reef Shark displaying to a much larger hammerhead, which subsequently
left the area. The ability to communicate, however minimally, across species
boundaries is no small feat (our own species is only moderately successful
at it). Its spectacular agonistic display enables the Grey Reef Shark to
resolve disputes with other sharks without resorting to violence which could
result in injuries to itself and thus interfere with its ability to feed or
mate.